While the legendaries this Gen were not up to par with Gen 4, we have some good ones. Meloetta is one of them. It's ability to take down any defensive foe (excluding Shuckle, Bronzong, and Dusknoir) makes it a valuable addition to OU, where teams can be using Blissey one minute, and Foretress the next. I have a few movesets for it.

Mixed.

Ability: Serene Grace

EVs: Attack, Sp Attack, Speed Nature: Hasty

Ancient Song: ALlows this Pokemon to be a mixed attacker.

Close Combat: A powerful Fighting attack for use with Step forme. Death to Blissey!

Shadow Ball: Deals with Ghost types, and has a good Sp Def lowering chance.

Focus Blast covers Dark types while Thunderbolt is alot more reliable and provides better coverage.Calm Mind to provide moar powah and Special Defense.Substitute to somewhat guarantee Calm Mind and for other obvious reasons.Psyshock STAB and because screw you, special walls.
*The Leftovers support Meloetta's Special Defense.

acrobatics - acrobatics is a good move because its power doubles if the user is holding no item. However, if the user is holding a flying gem it will be used before the move has time to calculate its power, so it triples!

Meloetta

Very difficult case. I recently got mine and I have two. I was planning that one will stay in Piruette Form while the other keeps its Aria Form, but turns out that if you use Relic Song during battle, Meloetta will reset anyway to Aria Form, which really does suck. My solution for the problem is that I will use one that will always be changing Formes, whenever I feel like it and the other that never will learn Relic Song.

Close Combat Extra-Mega-Ultra-Powerful in Piruette Form, but if it is needed, it can be used as coverage in Aria Form.

Shadow Claw:None of the other moves affect Ghost types (Psychic only works on Gengar) therefore this is a must.

You may notice that I have two Physical and two Special moves in the moveset above. In case you make an adjustment to it, keep two of each if you want a Form changing Meloetta. If not, try my other moveset.

Thunder Wave/Thunderbolt/Shadow Ball:Having the opponent paralyzed is pretty handy, especially that it also halves your opponent's speed (I recently found that out). Also if you miss with Focus Blast, they're less likely going to be able to take advantage of that. However Shadow Ball gives you the chance to hit most Ghost types effectively, which you can't really do with the other three. I can't decide. Help me out by posting some comments to this answer!